The present invention relates generally to magnetic resonance imaging (MRI) equipment and in particular to a method of providing electrically isolated power to patient monitoring equipment used in conjunction with MRI.
Magnetic resonance imaging can provide sophisticated images of the human body by detecting faint nuclear magnetic resonance (NMR) signals primarily from concentrations of hydrogen protons. In MRI, the patient is located in a strong polarizing magnetic field and hydrogen protons of the patient's tissues are excited into precession with a strong radio frequency (RF) pulse. A series of applied gradient magnetic fields are switched on and off to spatially encode the protons by phase and frequency after which sensitive antennas are used to detect the NMR signals which can then be reconstructed into images.
The MRI machine is a difficult environment for other electrical instrumentation. The switched magnetic gradients and the RF pulse create electromagnetic interference and the MRI receiving antenna is sensitive to interference from other devices. Magnetic materials such as those used in transformer and inductor cores must be eliminated from the region of the MRI machine because of the extremely strong polarizing magnetic field.
Often it is necessary to monitor physiological data of the patient in the MRI room before or during scanning. Such monitoring equipment may, for example, include blood pressure meters, anesthetic gas monitors, oximeters and ECG amplifiers requiring a source of electrical power.
If line power is used to power such monitoring equipment, care must be taken to isolate patient contacting portions of the equipment from the source of the line power, typically, 120 to 240 volts and 20 amperes or more. Conventional iron-core transformers can provide such isolation, but are impractical in the MRI environment because of their ferromagnetic cores. Ferrites, in contrast, can become saturated in the magnetic field of the MRI magnet, often 0.5 Tesla or more. Air core transformers, such as provide adjacent windings of copper conductors without a ferromagnetic core, may be used; however, air core transformers are bulky and inefficient, and this latter drawback generates heat that can be a problem in a patient contacting device.
U.S. Pat. No. 4,737,712 describes a method of providing isolated power to a patient contacting instrument by converting electrical power into another form (e.g. optical, mechanical, or acoustic power) and then reconverting those alternative forms of the electrical energy back into electrical energy for use by the device. The conversion process provides inherent limitations in power transfer. This approach requires complex mechanisms and is practically limited to extremely low power transfer.